Low-monomer content polyisocyanates have been used for some decades as useful hardeners for polyurethane coatings and adhesives.
These hardeners are generally prepared from diisocyanates. The modification of these diisocyanates to give polyisocyanates is particularly advantageous for two predominant reasons. Firstly, mainly branches are introduced by the modification, such that the polyisocyanates usually have NCO functionalities >2, often in the range of 2.4 to 4.5, and so are particularly well suited to the formation of highly cross-linked, very durable coatings. However, even for a solely linear modification, such as the reaction with diols to give linear prepolymers, the hardeners obtained are “given” particular properties, such as the ability to achieve particularly flexible coatings. Secondly, following the actual modification of the diisocyanates, in many cases the excess monomeric diisocyanate is removed and fed back into the modification process. The polyisocyanates have a vapour pressure an order of magnitude lower than the quite volatile diisocyanates, such that they are distinctly less physiologically active and are considerably easier to handle from an occupational hygiene point of view.
Diisocyanates used industrially in large quantities are toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI) and dicyclohexylmethane diisocyanate (H12-MDI). In addition to these industrially available diisocyanates, further diisocyanates are available in industrial quantities. These are described, for example, in addition to the diisocyanates explicitly mentioned above, in Ullmann (Christian Six, Frank Richter, 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 10.1002/14356007.a14_611) and frequently have the character of specialty isocyanates for very particular applications.
Aromatic diisocyanates, in which the NCO groups are bonded directly to an aromatic ring, such as TDI and MDI, are distinguished from aliphatic diisocyanates having isocyanatoalkyl groups, such as HDI, IPDI and H12-MDI. For high-quality lightfast coatings, the latter are of particular interest since they afford non-yellowing colour-stable coatings.
The oligomerisation of diisocyanates to polyisocyanates is known and has been described many times; for example, see H. J. Laas et al. in J. Prakt. Chem. 336 (1994), 185ff. The oligomerization methods described generally differ in the selection of the diisocyanates used, the selection of the catalysts and the choice of the specific reaction conditions.
The diisocyanate is typically placed in the reaction vessel and pre-heated to, or to slightly below, the reaction temperature. The catalyst solution suitable for the trimerisation is then added continuously over a certain time period. After the reaction commences (“onset” of the reaction), the strongly exothermic reaction is normally cooled and, by regulating the metered addition of the catalyst and cooling, the reaction is conducted so that the desired target NCO content of the crude solution is achieved. By addition of a chemical stopper, catalyst still present is deactivated. The oligomeric polyisocyanate is separated from excess monomeric diisocyanate in a subsequent process step. This separation is generally effected by distillation in suitable apparatuses, preferably by multi-stage distillation including at least one thin-film distillation.
EP-A 0003765 describes the trimerisation of IPDI using quaternary hydroxyalkyl-substituted ammonium hydroxides as catalysts at reaction temperatures of 30 to 90° C.
This reaction procedure is unfavourable since catalyst salts which form during and after the reaction are not fully soluble at reaction temperatures below 90° C., particularly in the case of the cycloaliphatic diisocyanates used here and quaternary ammonium hydroxide compounds used as catalysts and the acidic chemical stoppers. These salts, which appear in the form of turbidity, interfere with the heat transfer by covering reactor walls and cooling surfaces of the heat exchangers and necessitate frequent cleaning of the apparatuses. The quaternary ammonium hydroxide compounds used as catalysts are themselves thermolabile and are broken down at elevated temperatures; see, for example, H. J. Laas et al. in J. Prakt. Chem. 336 (1994), 185ff. In this case, the anion of the salt is alkylated and cleared into tertiary amines. Some of the decomposition products are volatile and some form salts which are in turn no longer catalytically active.
The object of the present invention, therefore, was to provide a discontinuous (batch) or continuous process in which the trimerisation of cycloaliphatic diisocyanates, catalysed by ammonium hydroxide compounds, is carried out such that deposition of solids that interfere with the operation of the plant does not occur in the apparatuses. In particular, the specific process shall ensure a long operating time without disruptions and costly downtime and without necessitating frequent cleaning operations. Moreover, compared to the trimerisation at a temperature below 90° C., no significantly higher catalyst requirement should occur.